The investment casting process is used to create metal components, e.g. turbine blades, by pouring molten metal into a ceramic shell of the desired final shape and subsequently removing the ceramic.
The process is an evolution of the lost-wax process whereby a component of the size and shape required in metal is manufactured using a wax pattern die into which molten wax is injected. The wax pattern is then dipped in ceramic slurry to create a ceramic shell on the wax pattern. The wax is removed and the shell fired to harden it. The resulting ceramic shell has an open cavity of the size and shape of the final component into which the metal can be poured. The ceramic shell is subsequently removed, either physically or chemically.
In order to make a component e.g. an aerofoil blade, with internal cavities e.g. internal cooling channels, a ceramic core is required. This is manufactured separately and is placed inside the wax pattern die prior to wax injection.
After casting the metal in the ceramic shell, the ceramic core is removed e.g. leached with alkaline solution to leave the hollow metal component.
Ceramic cores are manufactured using a ceramic injection moulding process (CIM). A ceramic material, usually silica, is suspended in an organic, polymeric binder to create a feedstock. This feedstock is then injected into a die cavity of the required size and shape to create a “green” component comprised of the ceramic and binder components. The binder is subsequently thermally or chemically removed and the ceramic consolidated by sintering/firing at elevated temperatures; this gives the final ceramic core.
The core is usually supported during the firing process by placing it within a ceramic receptacle and surrounding it with an inert firing powder. This has the advantage of promoting controlled binder removal by wicking during the early stages of firing. The current method involves the steps of manually adding an amount of firing powder into a ceramic receptacle, placing one or more ceramic cores into the firing powder and manually adding further firing powder to cover the top of the core(s).
The homogeneity of the firing powder, the position of the core within the receptacle and the orientation of the core within the receptacle have all been shown to affect the dimensions of the final fired ceramic core and thus the dimensions of the cavity/channel within the final metal component.
Accordingly, there is a need for a method and an apparatus with which accurate and repeatable dimensions can be obtained for a ceramic core.